1. Field of the Invention
The present invention relates to a system for cooling and maintaining a print head used in an inkjet printer at a constant temperature. More particularly, the present invention relates to forming cooling channels in a heater chip of a print head and continuously pumping ink through these channels to maintain the heater chip at the proper operating temperature even when the printer is not actively printing.
2. Description of the Related Art
In recent years color inkjet printers have been developed for home and office use. These inkjet printers operate by using a heater chip to heat ink contained in firing (vaporization) chambers that open to a nozzle. Each firing chamber has individual heating elements associated therewith and when the printer processor determines that a particular chamber should fire, an electric current is applied to the associated heater element for that firing chamber. The ink in the firing chamber is then rapidly heated and a bubble of ink is deposited on the paper. The chamber is then refilled by a capillary feed mechanism from an ink reservoir.
During the printing process, the print head normally moves back and forth across the paper while printing. Since the print is moving while simultaneously printing, the timing of heating the firing chambers in the print head is critical to proper placement of the ink on the paper. When the print head operates for a period of time, heat tends to accumulate in the print head. When excessive heat accumulates, the firing chamber may prematurely eject the ink on the paper and the image may be distorted. In addition, excess heat may cause air bubbles to accumulate in the ink and prevent normal functioning of the jetting mechanism. Further, if the heat continues to accumulate then damage may occur to the heater chip itself, which would necessitate its replacement.
A mechanism that has been incorporated in inkjet printers to alleviate this problem has been to slow the printing speed and thereby reduce the firing rate for the individual firing chambers. Since more time elapses between repeated heating of the firing chambers, the heater chip maintains a constant temperature through simple radiation of the heat. Another approach has been to build in a time delay at the point at which the print head has completed one pass while the paper is moving to the next print position. Both of these solutions cause undesirable delays in printing.
Additional methods of maintaining the temperature of a heater chip at the proper level are disclosed in the prior art discussed below.
U.S. Pat. No. 5,619,236 to Keefe et al. describes a method of cooling a heater chip by using an edge feed rather than a center feed firing mechanism in which the ink passes along the back and around the edges of the substrate containing the heating elements. By allowing the ink to pass along the back and edges of the heater chip, heat may be drawn off from the heater chip by the ink.
U.S. Pat. No. 4,899,180 to Elhatem et al. discloses an inkjet print head in which a heater chip is used to heat ink and thereby discharge it onto paper. The device consists primarily of two silicon chips and temperature sensors. As shown in FIG. 4, the first silicon chip consists of a heater chip 30 and a second silicon chip acts as a channel chip 42 in which ink channels are etched therein. The ink channels lead directly to firing chambers and nozzles. Ink flows through the channel chip only when printing is actively taking place.
U.S. Pat. No. 4,994,826 to Tellier discloses a method of forming channels on a heater chip in which a film layer is placed on the chip and a portion of the film is then removed to form channels. A plate is then placed on the film to form closed channels and nozzle openings.
In all the foregoing patents some degree of cooling is achieved for the heater chip. However, in every case the cooling channels are not formed directly in the heater chip and in order for the cooling mechanism to operate the print head must be actively depositing ink on paper. Therefore, the cooling mechanism of the prior art is not particularly effective since the ink used in cooling does not come in close contact with the heat producing elements of the heater chip. More importantly, the cooling mechanism disclosed in the prior art only works when the printer is actually depositing ink on paper. At all other times there is no active cooling that takes place. Thus, it is still necessary to build in time delays and othervise slow the printing process in order to insure that the heater chip remains at the proper temperature.
Therefore, a need exists for a simple and highly effective mechanism to maintain a heater chip at a desired temperature even when the printer is not actively printing. With a mechanism that is highly effective and continuously operates, it would be possible to print at the printer's maximum speed and be assured that the heater chip is maintained at the proper temperature.